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Tire pressure mpg tests - tire inflation

Discussion in 'Gen III 2010+ Prius Technical Discussion' started by harbormaster, Jul 25, 2009.

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  1. harbormaster

    harbormaster New Member

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    Curious if anyone with the 2010's have adjusted tire pressure and tracked the mpg numbers?
    My car came with the Avid S33 tires (p195/65 r15)
    Had the following pressures after two weeks of driving (only 600 miles)
    front driver side - 35
    front passenger side - 34
    rear driver side - 34.5
    rear passenter side - 34.5

    I will bump them up to 36 front and 34 rear.
    Of course the car door has specs has 35 front and 33 rear.
    The tire max pressure is 44.

    I don't want to over inflate too much and cause wear.

    Our minimal miles so far (not using the computer) have been
    46.8 (computer est. was higher)
    48.9 (computer est. was higher)
    current computer reads 52.4 - I hope when I double check on the next fill up it support the computers optimistic est.

    Thoughts ?
  2. a64pilot

    a64pilot Active Member

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    I'd go to 44. I doubt that 1 psi would make a measurable difference. I assume if you do nothing at all, your mileage will improve as the car breaks in and you learn the car wouldn't it?
  3. jay_man2

    jay_man2 jay_man_also

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    I have the Bridgestone Ecopia's and they were at 40 all around at delivery. I bumped the fronts to 42 after a week of ownership. My mileage on the MFD has been 54.8, 58, 55.1, 51.4 (stopped driving like granpa) and now 54.2 through half a tank.

    The computer readout will always be 1-3 miles higher, with a wider spread the higher it reads.
  4. tumbleweed

    tumbleweed Senior Member

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    Inflating the tires to 42 or 44 psi will most likely not cause excess wear. Somewhere in the Gen2 threads is a picture Hobbit took of the tires that came off his '04 after a lot (don't remember how many) miles. He kept them at over 40 psi for all of those miles and they were still worn more on the outside edges that the middle. I didn't keep the original tires on my '05 that long but they were starting to show the same wear tendencies and I used 42 psi in all 4 corners.

    Higher tire pressures usually result in the following:

    A slightly firmer ride,
    Slightly decreased traction on dry pavement,
    Increased resistance to hydroplaning,
    Less rolling resistance and thus better fuel economy.

    Modern radial ply tires don't seem to get bowed out and wear in the middle when they have more air in them like bias ply tires did. The belts seem to keep the tread nearly flat.

    I will increase mine when my Gen3 arrives and post the results at several different pressures.
  5. charlieh

    charlieh Junior Member

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    I have run my 04 at 42/40 since I got the car, and the mpg is definitely better than running at 36/34. I have good tread left at 38,000 miles. I am running the Ecopias on my 2010 at 42/40 and I am not done with my first tank, but I am currently at 60.4 mpg (mostly city and some freeway). I think I will try 44/42 and see how that works.
  6. harbormaster

    harbormaster New Member

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    Thank you all for the great feedback. Just curious how noticable the ride changes with the higher pressure. I can't seem to find any car or tire warranty issues at first read of the manual.
    Anyone have any preference as to regular air vs nitrogen?
  7. tumbleweed

    tumbleweed Senior Member

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    I don't think it makes much difference in ride, actually I prefer the car to ride a bit firmer anyway. You just need to try it.

    There has been a lot of discussion on this site about nitrogen in tires. The consensus is that there is no advantage to using it in passenger car tires. After all the stuff we breath is already 78% nitrogen and that's good enough. Personally, if I buy tires at Costco and they want to put in nitrogen that's fine but I wouldn't pay someone extra to do it.
  8. harbormaster

    harbormaster New Member

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    I have been the same way on the nitrogen. Tires are often from Costco, just yesterday as a matter of fact on my sons 96 honda civic (still a young one only 230,000 miles - on its fourth deer though - looking like Marty Feldman)
    I have just increased the pressure on the Prius to 42/40.
    I will post results.
    thx
  9. Spartane

    Spartane Member

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    Just last night I bumped the air pressure up to 42/40 for the Ecopia EP20 tires on my 2010. I was worried about a rough ride, but that didn't seem to happen.

    I reset trip-B before my trip to work this morning, and the car registered 3.1L/100km (75 mpg US) versus the more typical 3.4L/100km (69 mpg US) that I had been getting. So it looks like my fuel economy might have jumped by as much as 10%. (Note that I'm not actually getting 75 mpg; I've found that my car's display is overly optimistic -- also by about 10%).

    An unexpected bonus; handling on the highway got better in that fewer micro-corrections in steering were needed to keep the car running in a straight line.
  10. MSantos

    MSantos EcoAccelerometry

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    The Ecopias on the 2010 will be running at 55+ psi in the front and 52 in the rear merely as a start.

    At these pressure levels, the wear is significantly lower and the tires run much cooler (as indicated in the TPMS). That's the way I always pressed the tires on all my hybrids and other than the slight loss of comfort over rougher surfaces, the MPG and the safety are decidedly higher.

    Cheers;

    MSantos
  11. Spartane

    Spartane Member

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    Tire Rack quotes the max inflation pressure at 44 psi.

    Tire Specs for Bridgestone Ecopia EP20

    Is there a danger associated with exceeding this rating? If not, then what is the purpose of the rating?
  12. Glider

    Glider New Member

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    Lots of people think that if you inflate to 5-10 PSI above the MAX PRESSURE spec on the sidewall, the tire is going to blow up! Tire burst pressures are usually at least 200-300 PSI. CAVEAT: This is for undamaged, nondefective tires!!

    see retread.org

    [SIZE=+1]Q.[/SIZE]At what point will a truck tire burst from overinflation?
    Most new or undamaged medium radial truck tires can withstand three to four times the recommended pressure before bursting. In fact, the rim usually fails before the tire. ITRA has conducted several burst strength tests on new and used medium radial truck tires with special reinforced rims. The tires were pressurized with water. Most recently, three new tires and 13 used tires were "burst tested" with the lowest pressure burst recorded at 300psi and highest at 540psi for an average of 420psi. All of the test tires failed in the bead area.


    Tire manufacturers probably won't release the data from their burst tests because of LIABILITY issues.

    There are many issues about high PSI, but getting a blow-out from running a few PSI over recommended (or even max) pressure (for undamaged tires) is not high on the list. For example, see my post on "Over-inflated tires are a bad idea" or other posts.

    The danger is much higher to cause a blow-out from LOW PRESSURE. Driving a few hundred miles at 10-15 PSI will do you in a lot faster than a few PSI overpressure.
  13. MSantos

    MSantos EcoAccelerometry

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    Indeed.

    And while tire manufacturers will never formally disclose the burst pressures, we do have industry insiders in the community informally stating that catastrophic passenger vehicle tires failures due to over-inflation are almost unheard of and that the statistical majority of tire blow-outs are due to under-inflated operation and excessive sidewall fatigue (tire hysteresis).

    Also, depending on the manufacturer, burst pressures of 160 psi to 200+ have also been mentioned but along with that, a defect on the tire due to a puncture or prior damage may lower those numbers dramatically.

    That is why many in the community will press their (44psi) tires well above 60 and even 80 PSI with no ill effects to note, other than the decidedly better FE and longer tire life... not to mention better handling and less hydro-planing in wet roads.

    Cheers;


    MSantos
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  14. a priori

    a priori Canonus Curiosus

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    How does the TPMS indicate cooler tires? I didn't know I could get this information. Can you pick it up on ScanGauge?
  15. MSantos

    MSantos EcoAccelerometry

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    Nope.

    I install and use an aftermarket direct TPMS system (from APC) which allows me to monitor the pressures and temperatures of each tire... in addition to setting the warning thresholds for pressure and temperature as I see fit.

    Sadly, and although still a big plus in the larger scheme of things, the OEM systems usually get a resounding thumbs down from me in this department. :(


    Cheers;

    MSantos
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  16. David Beale

    David Beale Senior Member

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    -Most- max pressure printed warnings on the sidewall are actually max. load warnings. So "Max. load 942 lbs at 40PSI" will be the format. It's a load warning, not a pressure warning. The wording has drifted into the pressure area. I suspect this is by accident. The purpose of the warning was always for max. load rating.

    Now I've seen -some tires- with both a max. pressure and a max load warning, each separate from the other. The max. pressure was always much higher than the one in the max. load warning.
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  17. hobbit

    hobbit Senior Member

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    Clarification: the tire picture I hung out there is *not* my own
    tire, but that of another Prius owner I encountered while dumpster-
    diving around MIT. My own Integrities were worn evenly all the way
    across, and I gave the whole set to someone who thought he could
    use them as spares.
    .
    _H*
  18. tumbleweed

    tumbleweed Senior Member

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    Thanks for the correction, I should have tried to find the thread.
  19. harbormaster

    harbormaster New Member

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    Well, we increased the tire pressures to 42/40 and have driven a number of days. The ride seems fine. mpg has increased to 52.8 as per the computer. A fill up today measured 52.
    We still only have 900 ish miles on the car so the gain could also well be the cars natural improvements and/or our improved driving methods.
    FYI.
    Anyone else seen similar numbers?
    we tend to drive 60/40 highway (75) and city (35-45 avg)
  20. MSantos

    MSantos EcoAccelerometry

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    Well... I wouldn't say I have. :redface:

    Anyway, the Prius III can be a monster FE machine and indeed the best money can buy, so yes. I would say you are onto a great start.

    By the way, my wife easily cracks the 70 MPGs on a regular basis now. She just loves the car.

    Cheers;

    MSantos
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